Solid state lighting devices such as light-emitting diodes (LEDs) are increasingly used in both consumer and commercial applications. Advancements in LED technology have resulted in highly efficient and mechanically robust light sources with a long service life. Accordingly, modern LEDs have enabled a variety of new display applications and are increasingly utilized for general illumination applications, often replacing incandescent and fluorescent light sources.
LEDs are solid state devices that convert electrical energy to light and generally include one or more active layers of semiconductor material (or an active region) arranged between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the one or more active layers where they recombine to generate emissions such as visible light or ultraviolet (UV) emissions or infrared (IR) emissions. A LED chip typically includes an active region that may be fabricated, for example, from silicon carbide, gallium nitride, gallium phosphide, aluminum nitride, gallium arsenide-based materials, and/or from organic semiconductor materials.
Solid state emitters may include lumiphoric materials (also known as lumiphors) that absorb a portion of emissions having a first peak wavelength emitted by the emitter and re-emit light having a second peak wavelength that differs from the first peak wavelength. Phosphors, scintillators, and lumiphoric inks are common lumiphoric materials. Light perceived as white or near-white may be generated by a combination of red, green, and blue (RGB) emitters, or, alternatively, by combined emissions of a blue LED and a lumiphor such as a yellow phosphor (e.g., YAG:Ce or Ce:YAG). In the latter case, a portion of the blue LED emissions pass through the phosphor, while another portion of the blue emissions is downconverted to yellow, and the blue and yellow light in combination are perceived as white. White light may also be produced by stimulating phosphors or dyes of multiple colors with a violet or UV LED source.
Emissions of a blue LED in combination with a yellow or green lumiphoric material may be near-white in character and referred to as “blue-shifted yellow” (“BSY”) light or “blue-shifted green” (“BSG”) light. Addition of red (or red-orange) spectral output from a red-emitting LED (to yield a “BSY+R” device) or from a red lumiphoric material (to yield a “BS(Y+R)” device) may be used to increase the warmth of the aggregated light output and better approximate light produced by incandescent lamps.
Color reproduction is commonly measured using color rendering index (CRI) or average color rendering index (CRI Ra). To calculate the CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference radiator (illuminant) and the test source. The CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. (R9 is one of six saturated test colors not used in calculating CRI, with R9 embodying a large red content.) The CRI and CRI Ra are used to determine how closely an artificial light source matches the color rendering of a natural light source at the same correlated color temperature (CCT). Daylight has a high CRI Ra (approximately 100), with incandescent bulbs also being relatively close (CRI Ra greater than 95), and fluorescent lighting being less accurate (with typical CRI Ra values of approximately 70-80).
The reference spectra used in color rendering index calculations were chosen as ideal illumination sources defined in terms of their color temperature. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. Thus, apparent colors of incandescing materials are directly related to their actual temperature (in Kelvin (K)). Practical materials that incandesce are said to have CCT values that are directly related to color temperatures of blackbody sources.
Aspects relating to the inventive subject matter disclosed herein may be better understood with reference to the 1931 CIE (Commission International de l'Eclairage) Chromaticity Diagram, which is well-known and of which a copy is reproduced in FIG. 1. The 1931 CIE Chromaticity Diagram maps out the human color perception in terms of two CIE parameters x and y. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus (BBL) (also known as the Planckian locus) obey Planck's equation: E(λ)=A λ−5/(eB/T−1), where E is the emission intensity, λ is the emission wavelength, T is the color temperature of the blackbody, and A and B are constants. Color coordinates that lie on or near the BBL (which embodies a curved line emanating from the right lower corner) yield pleasing white light to a human observer. The 1931 CIE Diagram includes temperature listings along the BBL, with these temperature listings showing the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants which produce light that is on or near the BBL can thus be described in terms of their color temperature. The white area proximate to (i.e., within approximately a MacAdam eight-step ellipse of) of the BBL and between 2,500 K and 10,000 K, is shown in FIG. 1.
The term “white light” or “whiteness” does not clearly cover the full range of colors along the BBL since it is apparent that a candle flame and other incandescent sources appear yellowish, i.e., not completely white. Accordingly, the color of illumination may be better defined in terms of CCT and in terms of its proximity to the BBL. The pleasantness and quality of white illumination decreases rapidly if the chromaticity point of the illumination source deviates from the BBL by a distance of greater than 0.01 in the x, y chromaticity system. This corresponds to the distance of about a MacAdam four-step ellipse, a standard employed by the lighting industry. A lighting device emitting light having color coordinates that are within a MacAdam four-step ellipse of the BBL and that has a CRI Ra greater than 80 is generally acceptable as a white light for general illumination purposes. A lighting device emitting light having color coordinates within a MacAdam seven- or eight-step ellipse of the BBL and that has a CRI Ra greater than 70 is used as the minimum standards for many other white lighting devices including compact fluorescent and solid state lighting devices. FIG. 2 illustrates MacAdam 2-step, 4-step, and 7-step ellipses for a CCT of 3200 K relative to a segment of the BBL (e.g., extending generally between 2900 K and 3500 K).
Quality artificial lighting generally attempts to emulate the characteristics of natural light. Natural light sources include daylight with a relatively high color temperature (e.g., ˜5000 K) and incandescent lamps with a lower color temperature (e.g., ˜2800 K). General illumination generally has a color temperature between 2,000 K and 10,000 K, with the majority of lighting devices for general illumination being between 2,700 K and 6,500 K. The white area proximate to (i.e., within approximately a MacAdam eight-step ellipse of) of the BBL and between 2,500 K and 10,000 K, is shown in FIG. 1.
The 1976 CIE Chromaticity Diagram, also well-known and readily available to those of ordinary skill in the art, maps human color perception in terms of CIE parameters u′ and v′. The 1976 CIE Chromaticity Diagram (also known as the (u′v′) chromaticity diagram) is reproduced at FIG. 3. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The 1976 CIE Chromaticity Diagram is similar to the 1931 Diagram, except that the 1976 Diagram has been modified such that similar distances on the Diagram represent similar perceived differences in color. Since similar distances on the 1976 Diagram represent similar perceived differences in color, deviation from a point on the 1976 Diagram can be expressed in terms of the coordinates, u′ and v′, e.g., distance from the point=(Δu′2+Δv′2)′1/2, and the hues defined by a locus of points that are each a common distance from a specified hue consist of hues that would each be perceived as differing from the specified hue to a common extent. Duv is a metric that quantifies the distance between a color point and a point on the BBL having the same CCT in the u′, v′ coordinate system. A negative Duv value indicates a color point below the BBL and a positive Duv value indicates a point above the BBL.
Luminous efficacy is a measure of how well a light source produces visible light, and represents the ratio of luminous flux to power (with the power being either radiant flux or total power consumed by a source, depending on the context). Wavelengths of light outside of the visible spectrum are not useful for illumination because they cannot be seen by the human eye. Moreover, the human eye exhibits greater response to some wavelengths of light than to others, even within the visible spectrum. Response of the human eye to light also varies with respect to the level of intensity of light.
It has been recently recognized that photosensitive retinal ganglion cells expressing the photopigment melanopsin is involved not only in circadian photoentrainment, but also in perceived brightness of light. Melanopsin photoreceptors are sensitive to a range of wavelengths and reach peak light absorption at blue light wavelengths around 480 nm. A “melanopic” spectral efficiency function has been determined to predict the sensitivity of melanopsin photoreceptors to polychromatic lights. FIG. 4 is a plot of a melanopic spectral efficiency function, expressed in (p) versus wavelength (nm), derived from experimentation performed on transgenic mice lacking rod and cone photoreception, and as described in al Enezi et al., “A ‘Melanopic’ Spectral Efficiency Function Predicts the Sensitivity of Melanopsin Photoreceptors to Polychromatic Lights,” J. Biological Rhythms, Vol. 26, No. 4, August 2011, 314-323. The curve of FIG. 4 involves weighting of spectral irradiance profiles (for a range of colored and broad-spectrum white lights) as according to spectral sensitivity of a family of putative opsin photopigments with a maximum response wavelength in a range of 400 to 550 nm, with data being fit with a Gaussian distribution peaking at 484 nm.
In animals, circulating levels of the hormone melatonin (also known chemically as N-acetyl-5-methoxytryptamine) vary in a daily cycle, thereby allowing the entrainment of the circadian rhythms of several biological functions. Melatonin is produced in humans by the pineal gland, a small endocrine gland located in the center of the brain. The melatonin signal forms part of the system that regulates the sleep-wake cycle by chemically causing drowsiness and lowering the body temperature. Melatonin is commonly released in darkness (roughly 4-5 hours before sleep), and its production is suppressed by exposure to light. The light-dependent character of melatonin release and suppression aids in falling asleep and waking up. Nighttime light exposure can increase body temperature, and enhance alertness and performance.
Circadian rhythm disorders may be associated with changes in nocturnal activity (e.g., nighttime shift workers), changes in latitude or changes in longitude (e.g., jet lag), and/or seasonal changes in light duration (e.g., seasonal affective disorder, with symptoms including depression). It is principally blue light (e.g., including blue light at a peak wavelength value between 460 nm to 480 nm, with some activity from about 360 nm to about 600 nm), that suppresses melatonin and synchronizes the circadian clock, proportional to the light intensity and length of exposure. Exposure to principally blue light (e.g., emitted by artificial light sources generating emissions with significant blue content) at times when melatonin is typically released, such as nighttime, can detrimentally suppress melatonin production and disrupt the normal circadian rhythm.
The art continues to seek improved solid state lighting devices that provide desirable illumination characteristics and are capable of overcoming challenges associated with conventional lighting devices.